Direct positive silver halide emulsions containing compounds which accept electrons and spectrally sensitize the emulsion



United States Patent 3,501,310 DIRECT POSITIVE SILVER HALIDE EMULSIONS CONTAINING COMPOUNDS WHICH ACCEPT ELECTRONS AND SPECTRALLY SENSITIZE THE EMULSION Bernard D. Illingsworth and Harry E. Spencer, Rochester, N.Y., assignors to Eastman Kodak Company, Rochester, N.Y., a corporation of New Jersey No Drawing. Filed Jan. 17, 1967, Ser. No. 609,761 Int. Cl. G03c N36, N U.S. Cl. 96106 11 Claims ABSTRACT OF THE DISCLOSURE Compounds which function both as electron acceptors and spectral sensitizers are incorporated in direct positive photographic silver halide emulsions fogged with the combination of a reducing agent and a compound of a metal more electropositive than silver.

This application is a continuation-in-part of our U.S. patent application Ser. No. 533,455, filed March 11, 1966, now abandoned.

This invention relates to photographic materials, and more particularly to improved direct positive photographic materials.

Direct positive materials have been prepared by adding to a silver halide emulsion reducing agent, a compound of a metal more electropositive than silver, and an electron acceptor. One of the principal advantages of such direct positive emulsions is that the high-light areas of the images obtained with these materials are substantially free from fog. However, known materials of this type have required blue light exposure. In other words, they have been substantially insensitive to longer wave length radiation, such as green and red radiation.

Accordingly, it is one object of our invention to provide improved direct positive photographic materials. Another object of our invention is to provide novel, spectrally sensitized photographic emulsions fogged with reducing agent and a compound of a metal more electropositive than silver. A further object of our invention is to provide novel improvements in such emulsions which feature use of a compound which functions both as an electron acceptor and as a spectral sensitizer. Other objects of this invention will be apparent from this disclosure and the appended claims.

In accordance with this invention, direct positive photographic silver halide emulsions which are fogged with a reducing agent and a compound of a metal more electropositive than silver, are improved by adding to the emulsion a compound which, (1) functions as an electron acceptor and (2) spectrally sensitizes the emulsion to radiation having wave length longer than about 480 me.

The preferred compounds of this invention are those organic compounds which, (1) have an anodic polarographic potential and a cathodic polarographic potential which, when added together give a positive sum, and (2) which spectrally sensitize photographic emulsions to radiation having a wave length longer than 480 m Advantageously, these compounds provide spectral sensitivity such that the ratio of minus blue relative speed to blue relative speed is greater than 7 and preferably greater than 10.

Organic compounds useful herein may be termed spectral sensitizing electron acceptors. An especially useful class of spectral sensitizing electron acceptors are trimethine cyanine dyes containing a 2-aromatically substituted indole nucleus attached by the 3-carbon atom thereice of to the methine chain. Dyes of this type are described by Coenen et al. in U.S. Patent 2,930,694, issued Mar. 29, 1960, and British Patent 970,601 (and corresponding Belgian Patent 630,911). A preferred class of spectrally sensitizing electron acceptors are dyes containing an imidazo[4,5-b]quinoxalinium salt moiety, such as the dyes disclosed in Brooker and Van Lare Belgian Patent 660,253, issued Mar. 15, 1965. In these dyes, the imidazo [4,5-b]quinoxaline nucleus is attached, through the 2- carbon atom thereof, to the methine chain.

Another class of highly useful dyes are those having the following formula.

Formula I wherein R R and R each represents an alkyl group, such as methyl, ethyl, propyl or butyl, or an aryl group such as phenyl; X represents an anion, such as chloride, iodide, bromide, p-toluenesulfonate, methyl sulfate, perchlorate, thiocyanate, sulfonate, bromide, etc., and Q represents the atoms necessary to complete a nucleus to form a trimethine cyanine dye, such as a 6-nitrobenzothiazole nucleus, an imidazo [4,5-b]quinoxaline nucleus or a pyrrolo[2,3-b]pyridene nucleus, e.g.,

R5 /R \N N wherein R R and R each represents a value selected from those given for R R and R A typical specific useful dye of this class is 1,1',3,3,3,3'-hexamethylpyrrolo [2,3-b1pyridocarbocyanine salt (e.g., perchlorate).

One useful group of trimethine cyanine dyes having Z-aromatically substituted indole nuclei have the following general formula:

Formula II wherein X has the meaning given above; A represents an aryl substituent, such as phenyl; R and R each represents an ancohol radical, e.g., an alkyl substituent (including substituted alkyl) and preferably containing from about 1 to 8 carbon atoms, such as methyl, ethyl, propyl, butyl or octyl, or sulfoalkyl such as sulfopropyl or sulfobutyl, sulfatoalkyl such as sulfatopropyl or sulfatobutyl, or carboxyalkyl such as carboxyethyl or carboxybutyl, and, Y represents a substituent selected from the group consisting of a hydrogen atom, an aryl group, such as phenyl, and alkyl (e.g., methyl, ethyl, propyl or butyl) or alkoxy (e.g., methoxy, ethoxy or propoxy) substituted phenyl groups, or a heterocyclic aromatic group, such as thiophene radical.

Symmetrical imidazo[4,5-b]qninoxaline trimethine cyanine dyes, wherein each nucleus is attached through the 2-carbon atom thereof to the methine chain, are useful 3 in the practice of this invention. Typical of such dyes are those having the following general formula:

Formula III wherein X, has the meaning given above; and R R R and R each represents a substituent such as alkyl, e.g., methyl, ethyl, propyl or butyl, and X has the meaning given above. Reference is made to Brooker and VanLare, Belgian Patent 660,253, issued Mar. 15, 1965, for other classes and specific examples of dyes containing an imidazo[4,5-b]-quinoxaline nucleus, which are useful herein.

The dyes described above are to be regarded as representative of the spectrally sensitizing electron acceptors useful herein. Any dye which has the polarographic halfwave potential and which imparts the spectral sensitivity specified herein can be used in this invention.

This invention will be further illustrated by Examples 1-5 below. These examples demonstrate the superior results obtained when spectral sensitizing electron acceptors in accordance with the invention are incorporated in direct positive emulsions fogged with reducing agent and a compound of a metal more electropositive than silver. Examples A, B, C and D are comparative examples and illustrate the poor results obtained with dyes suggested by the prior art for these emulsions. These poor results are characterized by slow speed and ineffective spectral sensitization.

A gelatin silver bromiodide emulsion (2.5 mole percent of the halide being iodide) and having an average grain size of about 0.2 micron is prepared by adding an aqueous solution of potassium bromide and potassium iodide, and an aqueous solution of silver nitrate, simultaneously to a rapidly agitated aqueous gelatin solution at a temperature of 70 C., over a period of about 35 minutes. The emulsion is chill-set, shredded and washed by leaching with cold water in the conventional manner. The emulsion is reduction-gold fogged by first adding 0.75 mg. of thiourea dioxide per mole of silver and heating for 60 minutes at 65 C. and then adding 7.5 mg. of potassium chloroaurate per mole of silver and heating for 60 minutes at 65 C. The fogged emulsion is divided into several portions and 0.87 millimole per mole of silver halide of each of the dyes listed in the following table is added to -a portion of the emulsion. Each portion is coated on a cellulose acetate support at a laydown of 100 mg. of silver and.400 mg. of gelatin per square foot. Each coating is exposed to a tungsten source on an Eastman Ib sensitometer. The emulsions are given a blue exposure through Wratten 35 and 38A filters and a minus blue exposure through a Wratten 16 filter. The exposed coatings are processed for 6 minutes at room temperature in Kodak D-19 developer, fixed, washed and dried with the following results.

In the above table, it will be seen that the spectrally sensitizing electron acceptors of this invention which (1) Dye Used Example:

1 1,1-dimethyl-2,2'-diphenyl-3,3-indolocarbocyan1ne bromide.

2 1,1-dimethyl-2,2,8-triphenyl-3,3-indolocarbocyanine perchlorate.

3 2,2-di-p-methoxyphenyl-l,1-dimethyl-3,3-

indolocarboeyanine bromide.

4 1,1,3,3-tetraethylimidazo-[4,5-b]quinoxalinocarboeyanine chloride.

5 5,5 dichloro-3,3-diethyl-6,6-dinitrothiaearbocyanine iodide.

A Phenosafrauine.

13..-- Pinacryptol yellow.

0.... 3,3-diethyl-fi,6-dinitrothiacarbocyanine chloride.

D Crystal violet.

The most useful concentration of spectral sensitizing electron acceptors in the fogged emulsions of the invention is from 50 to 2,000, and preferably 400 to 800 mg. per mole of silver. However, these concentrations are given solely by way of instruction; best concentrations vary widely depending on the particular compound used.

The electron acceptors employed herein, as noted above spectrally sensitize the emulsions of the invention to radiation having a wave length longer than about 480 me. That is, the spectral sensitivity impart-ed to emulsions is at least equal to and usually greater than the sensitivity of the emulsion to blue radiation (i.e., 400 to about 480 mu).

Any silver halide emulsion may be used in this invention. For example, silver halide emulsions such as silver chloride or silver bromide, and mixed silver halides such as silver bromoiodide, silver chlorobromide or silver chlorobromoiodide are operable. Particularly useful are silver bromo halide emulsions comprising more than 50 mole percent bromide. These emulsions are fogged with the combination of a reducing agent and a compound of a metal more electropositive than silver. Typical useful reducing agents include stannous salts, e.g., stannous chloride, hydrazine, sulfur compounds, such as thiourea dioxide, phosphonium salts such as tetra (hydroxy methyl) phosphonium chloride, and the like. Compounds of metals such as gold, rhodium, platinum, palladium and iridium are used preferably in the form of soluble salts such as auric chloride and (NHQ PdCI The spectrally sensitizing electron acceptors which give particularly good results in the practice of this invention can be characterized in terms of their polarographic halfwave potentials, i.e., their oxidation reduction potentials determined by polarography. Cathodic measurements can be made with a l l0 molar solution of the electron acceptor in a solvent, for example, methanol which is 0.05 molar in lithium chloride using a dropping mercury elec trode with the polarographic halfwave potential for the most positive cathodic wave being designated E Anodic measurements can be made with 1X10- molar aqueous solvent solution, for example methanolic solutions of the electron acceptor which are 0.05 molar in sodium acetate and 0.005 molar in acetic acid using a carbon paste of pyrolytic graphite electrode, with the voltametric half peak potential for the most negative anodic response being designated E,,. In each measurement, the reference electrode can be an aqueous silversilver chloride (saturated potassium chloride) electrode at 20 C. Electrochemical measurements of this type are known in the art and are described in New Instrumental Methods in Electrochemistry, by Delahay, Interscience Publishers, New York, N.Y., 1954; Polarography, by Kolthoff and Lingane, 2nd edition, Interscience Publishers, New York, -N.Y., 1952; Analytical Chemistry, 36, 2426 (1964) by Elving; and Analytical Chemistry 30, 1576 (1958) by Adams. Compounds Which can be employed as spectrally sensitizing electron acceptors in the practice of this invention include organic compounds having an anodic polarographic halfwave potential (E,,) and a cathodic polarographic potential (E which when added together give a positive sum.

Best results are obtained using about .05 to 40 mg. reducing agent per mole of silver halide, and 0.5 to 15.0 mg., metal compound (e.g., gold salt) per mole of silver halide. However, these concentrations may be varied over a considerable range to obtain desired results.

The novel emulsions of this invention may be coated on any suitable photographic supports, such as glass, film base such as cellulose acetate, cellulose acetate butyrate, polyester such as polyethylene terephthalate, paper, baryta coated paper, polyolefin coated paper, e.g., polyethylene or polypropylene coated paper, which may be electron bombarded to promote emulsion adhesion.

The emulsions of this invention may be dispersed in any photographic hydrophilic colloid, such as any of those referred to in US. Patent 3,039,873, column 13, and may contain any of the hardeners or plasticizers referred to in columns 12 and 13 of that patent.

As used herein, fogged refers to emulsions containing silver halide grains which produce a density of at least 0.5 when developed, without exposure, for minutes at 68 F. in developer D-K50 when the emulsion is coated at a silver coverage of 50 mg. to 500 mg. per square foot.

Although the invention has been described in considerable detail with particular reference to certain preferred embodiments thereof, it will be understood that variations and modifications can be effected within the spirit and scope of the invention as described hereinabove, and as defined in the appended claims.

We claim:

1. In a direct positive photographic silver halide emulsion the halide of which comprises more than 50 mole percent bromide, which emulsion is fogged with the combination of reducing agent and a compound of a metal selected from the group consisting of gold, rhodium, platinum, palladium and iridium: the improvement which comprises adding to said emulsion an organic compound which (1) is an electron acceptor having an anodic polarographic potential and a cathodic polarographic potential which, when added together, give a positive sum and (2) spectrally sensitizes the emulsion to radiation having a wavelength longer than about 480 me so that the ratio of relative minus blue speed to relative blue speed of the emulsion is greater than 7.

2. In a direct positive photographic silver halide emulsion the halide of which comprises more than 50 mole percent bromide, which emulsion is fogged with the combination of reducing agent and a gold compound: the improvement which comprises adding to said emulsion an organic compound which (1) is an electron acceptor having an anodic polarographic potential and a cathodic polarographic potential which, when added together, give a positive sum and (2) spectrally sensitizes the emulsion to radiation having a wavelength longer than about 480 m so that the ratio of relative minus blue speed to relative blue speed of the emulsion is greater than 3. In a direct positive photographic silver halide emulsion the halide of which comprises more than 50 mole percent bromide, which emulsion is fogged with the combination of reducing agent and a compound of a metal selected from the group consisting of gold, rhodium, platinum, palladium and iridium: the improvement which comprises adding to said emulsion a dye which (1) is an electron acceptor having an anodic polarographic potential and a cathodic polarographic potential which,

when added together, give a positive sum and (2) spectrally sensitizes the emulsion to radiation having a wavelength longer than about 480 m so that the ratio of relative minus blue speed to relative blue speed of the emulsion is greater than 7, said dye being a cyanine dye containing an irnidazo[4,5-b]quinoxaline nucleus attached, through the 2-carbon atom thereof, to the methine chain.

4. In a direct positive photographic silver halide emulsion the halide of which comprises more than 50 mole percent bromide, which emulsion is fogged with the combination of reducing agent and a gold compound: the improvement which comprises adding to said emulsion a dye which l) is an electron acceptor having an anodic polarographic potential and a cathodic polarographic potential which, when added together, give a positive sum and (2) spectrally sensitizes the emulsion to radiation having a wavelength longer than about 480 III/1. so that the ratio of relative minus blue speed to relative blue speed of the emulsion is greater than 10, said dye being a cyanine dye containing an imidazo [4,5-b] quinoxaline nucleus attached, through the 2-carbon atom thereof, to the methine chain.

5. In a direct positive photographic silver halide emulsion the halide of which comprises more than 50 mole percent bromide, which emulsion is fogged with the combination of reducing agent and a gold compound: the improvement which comprises adding to said emulsion a dye which 1) is an electron acceptor having an anodic polarographic potential and a cathodic polarographic potential which, when added together, give a positive sum and (2) spectrally sensitizes the emulsion to radiation having a wavelength longer than about 480 m so that the ratio of relative minus blue speed to relative blue speed of the emulsion is greater than 10, said dye having the following wherein R7, R R and R each represents an alkyl radical; and, X represents an anion.

6. In a direct positive photographic silver halide emulsion the halide of which comprises more than 50 mole percent bromide, which emulsion is fogged with the combination of reducing agent and a gold compound: the improvement which comprises adding to said emulsion a dye which (1) is an electron acceptor having an anodic polarographic potential and a cathodic polarographic potential which, when added together, give a positive sum and (2) spectrally sensitizes the emulsion to radiation having a wavelength longer than about 480 m so that the ratio of relative minus blue speed to relative blue speed of the emulsion is greater than 10, said dye being a l,1',4,4-tetraethylimidazo[4, S b] quinoxalinocarbocyanine salt.

7. In a direct positive photographic silver halide emulsion the halide of which comprises more than 50 mole percent bromide, which emulsion is fogged with the combination of reducing agent and a gold compound: the improvement which comprises adding to said emulsion a dye which (1) is an electron acceptor having an anodic olarographic potential and a cathodic polarographic potential which, when added together, give a positive sum and (2) spectrally sensitizes the emulsion to radiation having a wavelength longer than about 480 m so that the ratio of relative minus blue speed to relative blue speed of the emulsion is greater than 10, said dye being a 1,1',3,3,3',3-hexamethylpyrrolo[2,3-b]pyridocarbocyanine salt.

8. In a direct positive photographic silver halide emulsion the halide of which comprises more than 50 mole percent bromide, which emulsion is fogged with the combination of reducing agent and a gold compound: the improvement which comprises adding to said emulsion a dye which (1) is an electron acceptor having an anodic polarographic potential and a cathodic polarographic potential which, when added together, give a positive sum and (2) spectrally sensitizes the emulsion to radiation having a wavelength longer than about 480 m so that the ratio of relative minus blue speed to relative blue speed of the emulsion is greater than 10, said dye having the following formula:

wherein R R and R each represents a substituent selected from the group consisting of alkyl and aryl; X represents an anion; and, Q represents the atoms necessary to complete a nucleus to form a trimethine cyanine dye.

9. In a direct positive photographic silver halide emulsion the halide of which comprises more than 50 mole percent bromide, which emulsion is fogged with the combination of reducing agent and a compound of a metal selected from the group consisting of gold, rhodium, platinum, palladium and iridium: the improvement which comprises adding to said emulsion a dye which l) is an electron acceptor having an anodic polarograp-hic potential and a cathodic polarographic potential which, when added together, give a positive sum and (2) spectrally sensitizes the emulsion to radiation having a wavelength longer than about 480 mg so that the ratio of relative minus blue speed to relative blue speed of the emulsion is greater than 7, said dye being selected from the group consisting of the dimethine and trimethine cyanine dyes containing a 2- aromatically substituted indole nucleus attached, by the 3-carbon atom thereof, to the methine chain.

10. In a direct positive photographic silver halide emulsion the halide of which comprises more than 50 mole percent bromide, which emulsion is fogged with the combination of reducing agent and a gold compound: the improvernent which comprises adding to said emulsion a dye which (1) is an electron acceptor having an anodic polarographic potential and a cathodic polarographi-c potential which, when added together, give a positive sum and (2) spectrally sensitizes the emulsion to radiation having a wavelength longer than about 480 m so that the ratio of relative minus blue speed to relative blue speed of the emulsion is greater than 10, said dye being selected from the group consisting of the dimethine and trimethine cyanine dyes containing a Z-aromatically substituted indole nucleus attached, by the B-carbon atom thereof, to the methine chain.

11. In a direct positive photographic silver halide emulsion the halide of which comprises more than 50 mole percent bromide, Which emulsion is fogged with the combination of reducing agent and a gold compound: the improvement which comprises adding to said emulsion a dye which (1 is an electron acceptor having an anodic polarographic potential and a cathodic polarographic potential which, when added together, give a positive sum and 2) spectrally sensitizes the emulsion to radiation having a wavelength longer than about 480 m so that the ratio of relative minus blue speed to relative blue speed of the emulsion is greater than 10, said dye having the following formula:

I Rn 1 1 12 X wherein A and A each represents an aryl substituent; R and R each represents an alykl radical; and X represents an acid anion.

References Cited UNITED STATES PATENTS 2,930,694 3/1960 Coenen et a1 96-105 X FOREIGN PATENTS 723,019 2/1955 Great Britain.

NORMAN G. TORCHIN, Primary Examiner R. E. FIGHTER, Assistant Examiner US. Cl. X.R. 96-101 

